In commonly owned U.S. Pat. No. 3,095,794 there has been described a camera of this type in which a handle is shiftable in an objective housing between a "manual" position and an "automatic" position; a camming sleeve for the adjustment of the overall focal length is coupled with a reversible drive motor in the "automatic" position and is decoupled from that motor in the "manual" position in which the sleeve can be rotated by the handle. In a later U.S. Pat. No. 3,224,351, a rotary knob can be similarly shifted on an objective housing for selective manual or automatic adjustment. In both these instances the manual/automatic selector, i.e. the handle or knob, is decoupled from the sleeve in the "automatic" position and therefore does not participate in its rotation by the motor; thus, the selector position is not related to focal length and a separate indicator must be provided to inform the user of the position last reached by the camming sleeve.
German Pat. No. 2,345,078 describes a varifocal objective or zoom lens in which the drive motor rotates the camming sleeve through a gear train, acting as a step-down transmission, and can be decoupled therefrom by the separation of two gears of that train. Upon such decoupling, the sleeve can be manually rotated by a lever or the like not illustrated in that German patent. Since the manual adjustment entails the entrainment of several meshing gears of the step-down transmission, the user must overcome an added frictional resistance. Also, because of the unavoidable play of the gear teeth, a false impression of a defective mechanism may be created.